This invention relates generally to the production of composite metal pipes having laminated multiple walls (hereinafter referred to as double-wall pipes) and used as chemical plant pipes, oil and gas well pipes, oil and gas line pipes, heat-exchanger tubes, and the like. More particularly, the invention relates to a method of producing double-wall pipes in which an inner pipe at one temperature is inserted in an outer pipe at another temperature, and, with these pipes in this state, pressure is applied to the interior of the inner pipe thereby to cause it to expand and to conformably adhere and be tightly joined to the outer pipe when both pipes assume substantially the same temperature.
Multiple-wall pipes such as double-wall pipes are being adopted for use as piping in chemical plants, oil and gas wells, and oil and gas pipelines, and as tubing in heat exchangers, apparatuses, and the like for the purpose of improving performance in preventing rust and withstanding corrosion. This trend is increasing with the recent development of the technology in fluid transportation.
Among the pipes of this character known heretofore for transporting fluids such as those containing corrosive substances, there is a double-wall pipe in which the inner pipe or liner tube is made of a corrosion-resistant material for effective handling of the corrosive fluid, while the outer pipe is designed to provide strength to withstand internal pressures. For example, there is a double-wall pipe comprising a stainless-steel inner pipe and a carbon-steel outer pipe in which the inner pipe is fitted.
Double-wall pipes of this character in which the inner and outer pipes are joined together with ample tightness are being developed.
The reasons for the necessity of ample tightness of joint, in general, in a double-wall pipe of this type may be considered to be as follows. One reason is that, in general, the temperature of a fluid in the interior of the pipe is different from that of a fluid outside of the pipe in many cases, and, in the case where it is desirable to reduce this temperature difference as much as possible by imparting high heat conductivity between the inner and outer pipes, it is preferable that the inner and outer pipes be adhering to each other with maximum possible force.
Another reason for the necessity of a tight joint is that, since the inner and outer pipes are made of different materials, in general, and have different coefficients of thermal expansion, in the case where temperature variations occur in the inner and outer pipes, the difference in the coefficients of thermal expansion or thermal contraction will tend to give rise to trouble such as slippage between the inner and outer pipes, local buckling, stress concentration, and fatigue rupture unless the two pipes are joined suitably with ample tightness thereby to cause the inner and outer pipes to behave as an integral structure.
As production methods for obtaining such a tight joint between the inner and outer pipes, the thermal shrink-fit method and the pipe-expanding method or hydraulic-expansion method are known. In the shrink-fit method, the outer pipe, the diameter of which is smaller than that of the inner pipe when both pipes are at the same temperature, is heated to be enlarged and the inner pipe is inserted therein. Then it is allowed to cool and shrink thereby to be tightly fitted onto the inner pipe. In the hydraulic expansion method, the inner pipe is inserted in the outer pipe and filled with a liquid, which is then pressurized to cause the inner pipe to expand and undergo a plastic deformation to be fitted tightly in the outer pipe.
However, both of these methods are accompanied by certain problems in the fabrication of the double-wall pipe and in their performance during use as described below and hereinafter.
In the thermal shrink-fit method, fundamentally, there are tolerances in the thicknesses of the inner and outer pipes, and the existence of out-of-roundness cannot be avoided with the pipes are produced by an ordinary process. Consequently, it is impossible in actual practice to carry out joining by the thermal shrink-fit method with a temperature difference of 400.degree. to 500.degree. C. with pipes in the asdelivered state. One measure which would appear to be possible for overcoming this problem is to machine the cylindrical outer and inner surfaces to be joined of the inner and outer pipes beforehand by a process such as machine grinding and polishing and then to carry out the thermal shrink fitting. This measure, however, is disadvantageous because of the required great labor and the resulting high cost.
In the case of pipe of long unit lengths, particularly those of thin walls, it is very difficult technically to obtain such a high precision in its dimension uniformly over the length of each pipe, and the production cost would be remarkably high.
Another problem encountered in this thermal shrink-fit method is the necessity of maintaining the temperature difference between the pipes uniform in the longitudinal direction of the pipes during this process. This method becomes difficult again in the case of pipes of long unit lengths and is ordinarily limited to the production of pipes of short unit lengths.
In the above mentioned hydraulic expansion method, as the pressure in the inner pipe is increased, the inner pipe expands until it contacts the outer pipe, and then, as the pressure is further increased, the two pipes are expanded unitarily until the stress in the outer pipe is near its yield point. The pressure is then reduced, whereupon both pipes shrink or contract elastically and, if they were free or independent of each other, would assume respective free states. If, in these free states, the outer diamenter of the inner pipe is greater than the inner diameter of the outer pipe, the tightening effect will be positive, and the two pipes will be securely joined together as desired. However, if, in these free states, the outer diameter of the inner pipe is less than the inner diameter of the outer pipe, the tightening effect will be negative, and there will be no tight joining of the pipes.
Whether or not a positive tightening effect will be produced depends on the mechanical properties of the materials and the sizes of the two pipes as will be described more fully hereinafter in conjunction with drawings including graphs. Thus, this hydraulic expansion method is subject to restriction of materials and their combinations.